Electrical current transducer modules for current sensing applications typically comprise a magnetic core made of a high permeability magnetic material, surrounding a central aperture through which passes a primary conductor carrying the current to be measured. The magnetic core may typically have a generally rectangular or circular shape and be provided with an air-gap in which a magnetic field detector, such as a Hall effect sensor in the form of an ASIC, is positioned. The magnetic flux generated by the electrical current flowing in the primary conductor is concentrated by the magnetic core and passes through the air-gap. The magnetic field in the air-gap is representative of the primary current. In current transducers of the open-loop type, the magnetic field sensor in the air-gap generates an image of the current to be measured that represents the measurement signal.
In current sensors of the closed-loop type the magnetic field sensor is connected in a feed-back loop to a secondary coil that is typically wound around a portion of the magnetic core in order to generate a compensation current that tends to cancel the magnetic field generated by the primary conductor. The compensation current thus represents an image of the current to be measured. In order to decrease the current required to drive the compensation coil, it is well known to wind a large number of turns around a section or most of the circumference of the magnetic circuit.
Typically, the compensation conductor coil is wound around an insulating bobbin support that surrounds a portion or whole of the magnetic circuit along which the secondary coil extends. The insulating bobbin is typically made of two parts assembled around the magnetic core, sometimes with an adhesive tape wound therearound. The insulating bobbin protects the first layer of the coil from damage and from being short circuited by the magnetic core, in particular during the winding manufacturing process and thereafter in use due to vibration, shock or thermal dilatation effects. The need for protection is particularly important for square or rectangular cross-section profiled magnetic cores in view of the sharp corners. Conventional insulating bobbins however add to manufacturing and assembly costs. Another drawback is the additional thickness of conventional two-part insulating bobbins that increases the diameter of the windings of the coil thus increasing the weight and length of copper wire used.
One problem linked to the protection of the magnetic core is that its geometry may be quite coarse. Indeed, inner and outer diameters and height can vary in a non-negligible manner. The coil obtained after winding depends on initial core dimensions. Another problem is due to winding effect: the wire tension force is quite important and the force applied on the protection is very local (important pressure stresses on a very small contact surface between winding wire and the protection, mainly on the first winding layer). Sometimes, wire repartition around the magnetic core while winding is not homogeneous due to the fact that initial layer of wires slides on the magnetic core protection. Further, in order to minimize copper wire necessary to obtain the different coils, the number of jumps during winding should be as low as possible. Often, 3 jumps corresponding to 3 fixing points with main PCBA are used.
Fixing the circuit board and secondary coil is often done by means of screws. This method is mechanically safe but it is not cost effective (additional parts and labor costs) and is risky (screw-driving machine head near PCBA electronic components). The link between the two parts has to be strong enough to permit the mounting of the transducer. The final mechanical fixing is in fact ensured by potting resin.
Positional accuracy of certain components in the transducer over its lifetime usage is also important to ensure reliability and measurement accuracy. In particular, the magnetic field detector should be precisely referenced into the magnetic core air gap in order to obtain good measuring head performance level. A further problem encountered in conventional transducers is that electrostatic screens are floating and badly referenced before potting process in the transducers.
Electrical current sensors are used in a large variety of applications for monitoring or controlling electrical devices and system and in many applications there is an important advantage in reducing the manufacturing cost of such components and also the costs of implementing and using the components in an electrical circuit. There is often also an important advantage in providing compact components in order to miniaturize and/or reduce the weight of the devices in which the components are mounted.